The long-term objectives of this competing continuation application are to: (1) Develop a compact, Cr4+- based tunable laser and optical fiber beam delivery system suitable for in vivo laser tissue welding animal studies, with potential for translation into clinical settings; (2) Develop a detailed understanding of the molecular mechanism behind LTW that includes the roles of thermal and non-thermal processes; (3) Improve welding efficacy by optimizing laser parameters, in particular, laser pulse-width, heat management, and buckling control; and (4) Evaluate the effect of physiological processes on welding and wound healing by extending LTW to living tissues using an animal model, and develop appreciation of its clinical potential and scope. To accomplish the long-term objectives, the specific aims of the research are to: (1) Develop a compact, all-solid-state laser pumped continuous wave (CW) Cr4+:YAG laser and modelock it to generate picosecond and femtosecond pulses, as well as, an optical fiber system to deliver the laser beam to the target.tissue to be welded; (2) Investigate changes in hydration and structure of collagen and elastin in ex vivo tissues as a result of welding, using Raman spectroscopy; polarized transmission (birefringence); histology; tensile strength analysis, and scanning and transmission electron microscopy; (3) Compare effects of laser pulse width (ps, fs and CW), heat management, and control of buckling on welding efficacy to determine whether short pulses can reduce collateral damage and improve bond strength in comparison to thermal processes; and (4) Extend the experimental approach, expertise, and knowledge of key LTW parameters to a carefully chosen animal model - albino guinea pigs - to investigate in vivo the wound healing response following LTW. The healing process and efficacy of welding will be evaluated by monitoring the degree and rate of epithelialization, extent of damage, granulation and collagen deposition, thickness of necrotic tissue sloughed off, degree of tissue apposition, and tensile strength of the welds on guinea pigs, over a 42-day period. The development of leak-proof, sutureless methods of healing surgical incisions will reduce risks of post-operative infection and scarring and provide an improved methodology for wound closure in microsurgery and endoscopic surgery. In addition, the use of ps and fs lasers for LTW is expected to result in reduced collateral damage and stronger tissue fusion. The outcome of the proposed research will have profound impact on wound healing and plastic surgery.